1. Technical Field
The present invention relates to a supply piping system for ultra-pure water to be used in large quantity in electronics industry for semiconductor products, biotechnology and pharmaceutical and medical industry.
2. Technical Background
The system according to this invention is applied widely in various industrial fields. Here, description will be given on the ultra-pure water piping system for electronics industry including the manufacture of semiconductor integrated circuits.
In recent years, ultra-pure water for electronics industry is produced by ultra-pure water producing system as shown in FIG. 3, and water is mostly supplied to the equipment to use ultra-pure water (may be called "use point" hereinafter, referring to the place to use ultra-pure water) through a piping system. In FIG. 3, an ultra-pure water supply piping system having 3 use points is shown as an example. In normal system, the number of the use points is far more than this. In FIG. 3, the primary pure water producing system is not shown.
In the following, description is given on the ultra-pure water manufacture and ultra-pure water supply to the use points in connection with FIG. 3. Key points to maintain water quality of ultra-pure water are the removal of dust and solid particles, removal of bacteria, and complete removal of various ions, silica, organic substances, etc. dissolved in water. In FIG. 3, primary pure water is supplied to a circulating water tank 102 through a primary pure water supply pipe 101. The primary pure water stored in the circulating water tank is pressurized by the pressure pump 103 and is purified by passing through an ultraviolet sterilizing unit (UV sterilizing unit) 104, a cartridge polishers 105, 106 and 107 and ultrafiltration units 108 and 109. 122 is a drain pipe of the ultrafiltration unit. The ultra-pure water thus purified is supplied to the use points 119, 120 and 121 through outward pipe 110, connection pipes 113, 114 and 115, and branching valves 116, 117 and 118 respectively. The ultra-pure water passing through the connection pipes 113, 114 and 115 are returned to the circulating water tank 102 through the inward pipe 112, and the circulation is repeated.
FIG. 4 is an emphasized view of an ultra-pure water supply piping system of FIG. 3. 201 comprehensively shows the ultra-pure water producing system. 131, 132 and 133 are the branching points of the connection pipes 113, 114 and 115 to the use points. The branching valves 116, 117 and 118 are integrated with the branching points 131, 132 and 133 respectively.
For the piping material, macromolecular materials such as clean vinyl chloride, PVDF, PEEK, etc. having circular and smooth surface and containing few contaminants in ultra-pure water are used. As the piping material with lowest dissolving ratio of contaminants, stainless steel pipe is used, which is produced by electropolishing the inner surface, by cleaning and drying thereafter and having passive film through oxidation in ultra-high purity oxygen at 500.degree.-600.degree. C. for 5-10 hours--particularly the passive film mainly consisting of Cr.sub.2 O.sub.3. Above all, the surface oxidized at 550.degree. C. for more than 9 hours has excellent property.
The supply piping system to the use points are provided with an outward pipe and an inward pipe as shown in FIGS. 3 and 4, and the connection pipes to the use points are furnished between them. The reasons are as follows: In FIGS. 3 and 4, only 3 use points are shown, whereas the number of the use points are normally far more than that. Therefore, the quantity of ultra-pure water flowing in the outward pipe is gradually decreased, and the quantity of water in the inward pipe is gradually increased. However, regardless of the using condition of each use point, the predetermined quantity of ultra-pure water is supplied to the use points from each connection pipe. Namely, the same quantity of water flows in the connection pipe upstream of the outward pipe and in the connection pipe downstream of it, and the pressure difference between outward pipe and inward pipe of each connection pipe is the same regardless of the position of the connection pipe. This results in the decrease of both circular pressure in outward pipe and circular pressure in inward pipe as it goes toward downstream, whereas the pressure difference between them is maintained at constant level. Therefore, the ultra-pure water supplied to the use points should be only the water passing through the outward pipe. This is the so-called reverse return type ultra-pure water supply piping system. When ultra-pure water is supplied to the use points, the branching valves 116, 117 and 118 should be opened.
In the conventional type ultra-pure water supply piping system as described above, the outward pipe and the inward pipe of ultra-pure water supply piping system in a clean room for semiconductor manufacture are arranged in parallel to each other at the same height in an underfloor pit room or in a service area. For this reason, no means were provided to cause a pressure difference between outward pipe and inward pipe.
This causes the following problems: In FIG. 4, outward pipe 110 and inward pipe 112 are conventionally at almost the same height, and the connection pipes 113, 114 and 115 are composed of the pipes with the same inner diameter for upstream and downstream of the branching point. Further, when the branching valves 116, 117 and 118 are opened, the pressure at the branching points 131, 132 and 133 are approximately equal to the atmospheric pressure. Accordingly, in case the branching valve 117 is opened when ultra-pure water is used in the ultra-pure water using equipment 120, the pressure at the branching point 132 is approximately equal to the atmospheric pressure. On the other hand, the pressure of ultrapure water flowing in outward pipe and inward pipe is adjusted in such manner that it is by approximately 1 to 2 kg/cm.sup.2 higher than the atmospheric pressure. Therefore, there occurs a flow passing from outward pipe 110 to the branching point 132 and a flow from the inward pipe 112 to the branching point 132 in the connection pipe, and these flows are mixed together and are supplied to the ultra-pure water using equipment. Ultra-pure water flowing from inward pipe to the branching point 132 is the ultra-pure water, which has come through the other connection pipes 113 and 115 and the inward pipe. Thus, it contains more impurities than the ultra-pure water, which has come from outward pipe through the connection pipe 114-1 only, because impurities are eluted from piping materials.
For instance, when water quality is expressed by electric resistivity, which is an index for concentration of ionic impurities contained in ultra-pure water, the resistivity of ultra-pure water supplied to ultra-pure water using equipment 120 flowing from outward pipe 110 through the connection pipe 114-1 only is 18.2 M.OMEGA..cm, while the resistivity of water supplied to the ultra-pure water equipment 120 from inward pipe through the connection pipe 114-2 is sometimes about 18.0 M.OMEGA..cm. The difference of the resistivity 0.2 M.OMEGA..cm corresponds to the concentration difference of about 0.2 ppb if the impurities are sodium ions. In other words, conventional type ultra-pure water supply piping system is disadvantageous in that it cannot supply ultra-pure water of ultra-high purity.
As described above, it is not desirable that ultra-pure water containing a large quantity of impurities is supplied by ultra-pure water using equipment due to the reverse flow from inward pipe. It is an object of this invention to offer a piping system, which prevents the reverse flow from inward pipe to ultra-pure water using equipment and stably supplied ultra-pure water of ultra-high purity.